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  #1  
Old 02-17-2011, 09:07 AM
DaveUK DaveUK is offline
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Default Latency?

I often use PT HD systems to record, but I am looking at the possibility of recording with native systems. I am trying to understand the latency.

I just tested the latency on my 002 on various buffer and low latency settings, I don't understand why there is such a big difference between the playback engine H/W buffer size and the actual latency (Changing the DAE buffer size appeared to make no difference).

I measured the latency in two ways - with a mikeand percussive sound with the monitors feeding the signal back to give the most realistic scenario latency (and subtracting the time the sound takes to travel from the speakers to the mic) and also within PT - a source click sound chained via analogue outs and ins to a record channel and then on to another record channel. The results were consistent:

H/W buffer measured latency
32 samples 269 samples
64 samples 402 samples
128 samples 605 samples
256 samples 1117 samples
Low Latency 87 samples

Low latency seems pretty good, but why is there such a big difference between the H/W buffer size and the actual latency? Will this latency be a lot smaller with other (non-HD) interfaces?

Last edited by DaveUK; 02-19-2011 at 03:48 AM. Reason: I'd mistyped one of the figures in the table
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Old 02-17-2011, 12:28 PM
daeron80 daeron80 is offline
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Default Re: Latency?

Huh. I may need to test mine again. I tested my 002R in 2006 running whatever was current then - probably some flavor of 7.x. Came up with significantly less latency except with LLM, which is identical. See the results here.

What sample rate did you test at? That makes a big difference.

Latency does vary some with different interfaces. It's all in the design of the converters and the hardware buffer. The buffer takes some of the load off the CPU somehow. Not sure why, but it does. I believe H/W buffers are built into the interfaces themselves. The DAE buffer is probably a software thing related to how the engine talks to the CPU or something. Totally different animal.

Dang. Looks like I documented everything about my experiments except the version of PT I was running. I see from the mixing latencies page that I had EQ3, so that had to be at least 7.0, right?
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Old 02-17-2011, 01:00 PM
getz76 getz76 is offline
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Default Re: Latency?

@daeron80,

Heh, I had come accross your results before but never realized you were the author! I think I found the page via a Google search, and did not read the footer with your info.

Great write up.
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Old 02-17-2011, 01:28 PM
daeron80 daeron80 is offline
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Default Re: Latency?

Thanks!
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Old 02-18-2011, 05:07 AM
DaveUK DaveUK is offline
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Default Re: Latency?

Hi Daeron, I hadn't seen your site before, very clear and great information.

I ran the tests on OSX, with the session at 48KHz. The figures seem to be in the same ballpark as yours, but offset by one - ie my 32 sample measurement corresponds roughly to your 64 sample measurement.

My experiment (version one - not the mic test) seems to be similar to yours, except I measured the time by selecting a range visually zoomed in matching the first peak of my sample waveform (a cowbell sample) and took the readings from the sample ruler.
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Old 02-18-2011, 02:11 PM
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albee1952 albee1952 is online now
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Default Re: Latency?

Don't forget; the buffer setting is only part of the total latency. There is also the conversion latency(somewhere around 1.2ms for a round trip).
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Old 02-18-2011, 03:05 PM
getz76 getz76 is offline
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Default Re: Latency?

Quote:
Originally Posted by daeron80 View Post
Thanks!
I used your site (sent a link) to a guy I was in a band with, explaining that tracking latency was not really an issue. The problem with Pro Tools (at the time) was mixing delay compensation.

He was a really smart guy but the concept totally went over his head.
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Old 02-19-2011, 03:46 AM
DaveUK DaveUK is offline
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Default Re: Latency?

Quote:
Originally Posted by albee1952 View Post
Don't forget; the buffer setting is only part of the total latency. There is also the conversion latency(somewhere around 1.2ms for a round trip).
true. 1.2 ms is around 57samples I think though, and would be a constant added rather than multiplied with each buffer size.

In other words if the results were buffer size plus conversion delay + general system delay, we would expect the difference in latency to be only the number of extra samples added to the buffer.

ie with a buffer of 128 we have a latency of 605 samples; one might imagine this was buffer+system+conversion delay, ie 128+477. Increasing the buffer to 256 we would then expect 256+477=733, but actually I measured 1117, a lot more.

Its all academic really, the latency options are what they are, although I find the labels misleading. Do all DAWs have a similar sized discrepancy between buffer size and actual latency?
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